This invention relates to a cap, and a method for cap sealing. More specifically, the invention relates to a roll-on cap having a liner of a specified shape and a method for sealing a container using the cap.
In a conventional roll-on cap consisting of a metallic cap shell having a circular top and a skirt extending downwardly from the peripheral edge of the top and provided with a deformable shoulder and a thread-forming portion deformable along the thread of the opening part of a container, and a flexible liner provided inwardly of the top of the cap shell, the liner usually consists of a disc of paper or a synthetic resin such as polyvinyl chloride bonded to the top of the cap shell, or of a polyvinyl chloride plastisol flowed into the cap shell top.
The disc-shaped liner, however, has the defect that the cost of its production is high because of poor productivity, and the liner tends to drop off from the cap shell during transportation and sealing operations. On the other hand, the flow-in liner (spin liner) can be produced with high productivity. However, the polyvinyl chloride used as the material poses a problem of toxicity owing to the vinyl chloride monomer, and it is desired to avoid its use in caps for foodstuff containers.
Polyethylene has recently attracted attention as a liner material which can replace polyvinyl chloride. Unfortunately, polyethylene cannot be used to produce flow-in liners since it is difficult to convert to a plastisol and has poor adhesion to the cap shell.